Though over 100 minerals are known to contain the rare earth or lanthanide elements, only a few provide the bulk of the world source of these elements. The principal rare earth-containing mineral monazite, is recovered by gravity, magnetic, electro static, and flotation concentration methods. The principal rare earth source mineral in the United States is bastnasite (LnFCO.sub.3). Initial processing of bastnasite comprises crushing the ore, and then using a flotation process to separate the source associated quartz, barite, calcite and strontianite. The flotation process yields an ore concentrate containing up to about 60 weight percent rare earth calculated as rare earth oxides. The concentrate then is leached with dilute hydrochloric acid to remove some of the alkaline earth constituents. This produces a concentrate having a rare earth oxide value of approximately 70 weight percent.
The ore is next roasted in an oxidizing atmosphere to convert the fluorocarbonate mineral contained therein to an oxide. It also oxidizes the cerium to a tetravalent state, which is essential to the subsequent separation of cerium from the other rare earths. Typically, the roasting is accomplished in an oxidizing atmosphere at an elevated temperature.
The roasted ore is then leached with a dilute acid. In this leach the non-cerium rare earths are solubilized. The cerium oxide is nominally insoluble in dilute HCl; thus the two are readily separated. The soluble oxides of the rare earths are transferred to a solvent extraction facility for their separate recovery.
The insoluble residue remaining from the foregoing acid leach is contacted in a closed reactor with a concentrated hydrochloric acid which solubilizes the cerium oxide for later recovery in a high purity form. During this step, the cerium concentrate is typically contacted with a 50 weight percent hydrochloric acid which, in addition to dissolution of cerium oxide, produces chlorine which is purged from the reactor. The chlorine comprises a waste stream which is scrubbed with a sodium hydroxide solution to prevent environmental contamination. The scrubbing solution is converted to a sodium chloride containing solution. Thus, the chlorine is converted from a hazardous waste gas to a less hazardous aqueous waste. Moreover, this process requires a substantial expense for sodium hydroxide plus produces a liquid waste.